Vehicular single shaft gas turbine engine power system

ABSTRACT

A vehicular gas turbine engine power system includes an engine, an automatic clutch, a service clutch and an infinitely variable transmission coupled successively along a power train. A vehicle control system controls engine speed, transmission ratio and automatic clutch engagement in response to operator selected ground speed and engine speed commands as well as other vehicle conditions. The vehicle control system operates in a manual mode to maintain engine speed as commanded by an operator or in an automatic mode to maintain an engine speed which will minimize fuel consumption. The transmission ratio is controlled for fixed rate vehicle acceleration toward a commanded speed if sufficient power is available. Otherwise ground speed is cut back to match required power with available power. However, ground speed cut back is limited as a safety feature and disengagement of the automatic clutch prevents engine stall when the engine becomes overloaded.

This is a division of application Ser. No. 623,319 filed Oct. 17, 1975,now U.S. Pat. No. 4,109,772.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to single shaft gas turbine engine vehicle powersystems and more particularly to vehicle power systems with automaticengine speed and transmission ratio control.

2. Description of the Prior Art

A turbine engine having a power turbine rotor which is directly coupledfor rotation with a compressor rotor may be referred to as a singleshaft gas turbine engine. Such engines are used extensively for fixedspeed applications such as generation of electricity because of theirlow initial cost and superior reliability. However, the curverepresenting torque variations with respect to engine speed for suchengines has a steep, narrow peak. As a result, a single shaft gasturbine engine develops maximum torque and power at a particular enginespeed which is typically in the range of 50,000 to 70,000 R.P.M. Thisspeed at which maximum torque and power are developed is often referredto as 100% rated speed, and torque and power decrease rapidly as enginespeed increases or decreases away from the 100% rated speed.

Because of this torque-speed characteristic single shaft gas turbineengines have not been extensively used in vehicular applications where asubstantial and continuous range of operating speeds is required.However, a single shaft gas turbine engine may be utilized to advantagein a vehicular power system when coupled with an infinitely variabletransmission. With a proper control system, operation of such anarrangement may be maintained with the transmission ratio controlled toprovide engine operation at 100% rated speed under full load conditions.Under part load conditions the transmission ratio may be continuouslyadjusted for optimum part load fuel consumption irrespective of vehiclespeed. An automatic control system for such an arrangement was presentedto the Society of Automotive Engineers Mid-Year Meeting, Montreal,Quebec, Canada, June 7-11, 1971. The presentation has been published asSAE publication number 710551, "Controls for Single Shaft Gas TurbineVehicles," by Bernard E. Poore.

The present invention provides further improvements in power systems ofthe type therein described. For example, the energy required to start avehicular gas turbine engine is considerable. A single shaft gas turbineengine must typically be accelerated to about 55% of its rated speedbefore engine operation becomes self sustaining. At this speed theengine is rotating at several thousand R.P.M. and considerable kineticenergy is possessed by the rotating parts of the engine. The enginestarting system must supply not only this kinetic energy, but alsoenergy to overcome engine friction and energy to drive vehicleaccessories and their drive gears. Connection of vehicle accessories"behind" the service clutch would mean interruption of the accessorieseach time the service clutch is disengaged. This would be undesirableand unnecessary for accessories such as air conditioner compressor andperhaps unacceptable for accessories such as an alternator, an hydraulicpump or an air compressor. Furthermore, an automatically controlledturbine power system may be somewhat more subject to engine stall underheavy load conditions where a vehicle operator is anticipating automaticoperation and an overload condition causes engine stall before theoperator can act to relieve the overload. In the event of a stall aconsiderable delay is encountered as the engine is restarted, and, asexplained above, a considerable burden is placed on the starting system.

Another problem associated with known vehicular turbine power systems isan inadequate indication of vehicle operating conditions. In aconventional gasoline engine or diesel engine vehicle power system,there is a substantial relationship between engine speed and vehicleload. A tachometer indication is thus adequate for proper vehicleoperation. However, when a vehicle is powered by a turbine engine andinfinitely variable transmission having an automatic control systemthere may be little relationship between engine speed and vehicle load.Some further indication of vehicle load condition then becomesdesirable.

SUMMARY OF THE INVENTION

A vehicular gas turbine engine power system in accordance with theinvention includes a single shaft gas turbine engine providingrotational energy, an automatic clutch coupled to receive rotationalenergy from the engine and output the energy only when engaged, aservice clutch coupled to receive rotational energy from the output ofthe automatic clutch and output the energy when selectively engaged byan operator, an infinitely variable transmission coupled to receiverotational energy output from the service clutch and output rotationalenergy at a variable torque ratio for vehicle locomotion, and a vehiclecontrol system. The vehicle control system is coupled to control enginespeed, transmission ratio and automatic clutch disengagement in responseto engine and vehicle speed commands from an operator. A vehicle loadmeter indicates engine speed as well as vehicle load conditions with acontinuous range of indicated magnitudes.

The control system operates in a manual mode wherein an engine speedcontrol lever is advanced to maintain engine operation at a speedindicated by the lever. A transmission ratio is commanded which willmaintain vehicle speed as indicated by a vehicle speed lever, exceptthat vehicle speed is reduced if there is insufficient power availableat the commanded engine speed. In an automatic mode of operationindicated by placement of the engine speed lever in a neutral position,a transmission ratio is commanded which will maintain vehicle speed asselected by the speed control lever if sufficient power is available.Engine speed is controlled in response to exhaust gas temperature andengine speed feedback for operation at 100% rated speed under full loadconditions and for optimum fuel consumption under part load conditions.

The automatic clutch is controlled for automatic disengagement at enginespeeds below 55% rated speed and automatic engagement at engine speedsabove 55% rated speed. This engagement speed is selected as beingslightly greater than a stall speed below which engine operation can beself sustained but less than normal idle speed. A memory circuitprevents limit cycling under an overload condition by inhibiting clutchreengagement following disengagement until reset by a manual resetswitch. The reset switch may be conveniently implemented as part of astarter switch to permit an automatic reset at start-up. With essentialengine driven accessories such as a fuel pump or an oil pump coupledahead of the automatic cluch and non-essential accessories coupledbehind the automatic clutch, the load on the starting system can begreatly reduced during start-up by automatic disconnection of theunessential accessories. However, once a normal idle speed is attainedthe unessential accessories are driven in a conventional manner withoutinterruption. The automatic clutch also operates to prevent engine stallin the event an overload condition occurs. This prevents a long restartdelay as well as additional wear on the starter system.

A vehicle load meter operates in response to engine speed and vehiclespeed cut back to indicate vehicle load conditions with a displayindication which may increase continuously in magnitude throughout aplurality of load indication ranges. The indication may be provided by asimple meter having a pointer which is rotated in proportion to themagnitude of an input signal. In a lower range of indication part loadengine speed is indicated from 0 to 100% of rated speed. The meter thusfunctions as a conventional tachometer in the lower range of indication.As load demands on a vehicle are increased in an automatic mode ofoperation the engine operating speed is first increased to 100% ratedspeed. Then, while the engine is maintained at 100%, further loadincreases cause a reduction in transmission ratio to decrease the loadby decreasing vehicle speed until the load demand matches engine power.An intermediate meter range indicates this ground speed cutback bysumming a G1 ERROR signal indicating the difference between commandedand actual speed with an N1 engine speed signal which indicates 100%engine speed in this range of operation. the gain of the ground speederror signal is selected to cause the indicator to change from anindication of 100% rated speed at the lower end of the intermediaterange to maximum cutback at the upper end of the intermediate range asground speed cutback reaches 50% of commanded speed.

A safety feature of the control system becomes operative as ground speedcutback reaches 50% to prevent further reductions in the transmissionratio. This feature prevents an accidental setting of commanded groundspeed at more than twice the actual ground speed. Sudden and excessiveaccelerations are thus prevented upon termination of an overloadcondition. Beyond 50% ground speed cutback engine speed is reduced and anormally small engine speed error signal which controls fuel flow beginsto increase. A threshold circuit senses this increase to clamp theengine speed signal at 100%. The increased engine speed signal thuscauses the indicator to increase in magnitude through an upper range ofindication as engine speed cutback occurs. A 55% engine speed at whichautomatic overload clutch disengagement occurs may be marked on themeter scale in the upper range and a stall speed may be marked beyondthe automatic clutch point.

The vehicle load meter thus operates in an automatic mode to provide anindication of vehicle load condition which is much more useful than amere tachometer indication. In a manual mode of operation the meteroperates as a tachometer within the lower range of indication.

BRIEF DESCRIPTION OF THE DRAWINGS

A better understanding of the invention may be had from a considerationof the following detailed description, taken in conjunction with theaccompanying drawings, in which:

FIG. 1 is a block diagram representation of a single shaft gas turbineengine vehicular power system in accordance with the invention;

FIG. 2 is a schematic and block diagram representation of a vehicularcontrol system for use in the vehicular power system shown in FIG. 1;

FIG. 3 is a schematic representation of a circuit for an F1 functionalelement shown in FIG. 2;

FIG. 4 is a schematic representation of a circuit for an F2 functionalelement shown in FIG. 2;

FIG. 5 is a schematic representation of a circuit for an F3 functionalelement shown in FIG. 2;

FIG. 6 is a schematic representation of a circuit for an F5 functionalelement shown in FIG. 2;

FIG. 7 is a graphical representation of relationships which arepertinent to the control system shown in FIG. 2;

FIG. 8 is a schematic representation of a circuit for an F6 functionalelement shown in FIG. 2;

FIG. 9 is a schematic representation of a circuit for an F7 functionalelement shown in FIG. 2; and

FIG. 10 is a schematic and block diagram representation of a load meterfor use with the vehicular power system shown in FIG. 1.

DETAILED DESCRIPTION

As shown in FIG. 1, a vehicular single shaft gas turbine engine powersystem 10 in accordance with the invention includes a vehicular controlsystem 12 and an associated power train and operator interface elements.The power train includes respectively a single shaft gas turbine engine14, a planetary reduction gear assembly 16, an essential accessory geardrive assembly 18, an automatic clutch 20, a reduction and non-essentialaccessory gear drive assembly 22, a service clutch 24, and an infinitelyvariable transmission 26. The infinitely variable transmission 26 iscoupled to drive the vehicle wheels in a conventional manner, forinstance through a differential rear end. The operator interfaceelements include an engine speed control 28, a ground speed control 30,a load meter 32, a speedometer 34, and a reset switch 33.

The single shaft gas turbine engine 14 is characterized by a compressorand a single power turbine coupled for fixed ratio rotation about thesame shaft. Such an engine has a curve representing torque as a functionof engine speed with a rather steep, narrow peak. It thus becomesnecessary to operate the engine within a relatively narrow speed rangeto obtain substantial power from the engine. Engine speed is controlledin a conventional manner by regulating the engine fuel flow in responseto a fuel command signal which is normally proportional to thedifference between a commanded and an actual engine speed. As the fuelcommand signal increases in magnitude, the fuel flow rate is increasedand the engine 14 delivers more power. If load conditions will permit,the increased power causes the engine to accelerate until a reduction inthe N1 error signal commands a reduction in the fuel command signal andthus fuel flow rate. Three principal engine condition signals arecommunicated from the engine 14 to the vehicle control system 12. Theseinclude an intake air temperature signal (T1), an exhaust gastemperature signal (T5), and an engine tachometer or velocity signal(N1).

A planetary reduction gear assembly 16, which may be conventional innature, is coupled to receive high velocity rotational energy from theturbine engine 14 and provide a velocity reduction and correspondingtorque increase of approximately 5:1. The 100% speed velocities of about60,000-70,000 R.P.M. at the output of turbine engine 14 are reduced toapproximately 12,000-14,000 R.P.M. at the output of reduction gearassembly 16.

An essential accessory gear drive assembly 18 is coupled to receiverotational energy from gear reduction assembly 16 and generatemechanical energy for driving essential accessories. Power outputs tothe engine 14 to drive a fuel pump and a lubrication pump are shown byway of example. A drive system for power brakes or power steering mightbe another example of essential accessories. The accessories which aredriven by essential accessory drive gear assembly 18 are directly andcontinuously coupled to the engine 14 and are energized so long as theengine 14 is running.

An automatic clutch 20 is coupled to receive rotational energy fromessential accessory drive gear assembly 18 and output rotational energyto reduction and non-essential accessory gear drive assembly 22. Itshould be appreciated that the power train might readily be slightlymodified by directly coupling the automatic clutch 20 and the accessorygear drive assembly 18 to the planetary reduction gear assembly 16 byseparate parallel power paths rather than in series as shown. In eithercase, both the automatic clutch 20 and the essential accessory geardrive assembly 18 would be directly and continuously coupled to receiverotational energy from turbine engine 14. Automatic engagement anddisengagement of automatic clutch 20 is controlled by a clutch controlsignal generated by vehicle control system 12. It is anticipated thatautomatic clutch 20 should be continuously engaged under normaloperating circumstances. However, the clutch control signal may begenerated to disengage automatic clutch 20 during engine start-up andduring emergency overload conditions. For start-up, a turbine enginemust typically be accelerated to a velocity of approximately 55% of themaximum torque velocity before engine operation can be self-sustaining.The engine starting system must be able to supply a considerable amountof energy in order to overcome the load which results from the inertiaand friction of the rotating parts as this high speed is attained.Disengagement of automatic clutch 20 permits a reduction in thisinertial and frictional load by requiring the driving during start-up ofonly the planetary reduction gear assembly 16 through which the starteris typically coupled and essential accessories. The energy which must besupplied by the starting system is thus greatly reduced.

Automatic clutch 20 may also be disengaged to prevent engine stallduring emergency overload conditions. For example, it is anticipatedthat the vehicle control system 12 should monitor the engine operatingspeed as indicated by signal N1 and generate a clutch control signalwhich will cause clutch disengagement in the event that the engine speeddecreases to a speed which is very near the engine stall speed ofapproximately 55%. Disengagement of automatic clutch 20 removes theoverload condition and permits the engine 14 to accelerate withoutstalling. A normal delay of about 30 seconds and the attendant loaddemand on the starting system for an extra start-up is thus avoided. Inorder to prevent limit cycling in the event of an overload condition,once the automatic clutch is disengaged the vehicle control system 12inhibits further engagement until activation of a clutch reset signalprovided by switch 33. Switch 33 may be advantageously implemented aspart of the normal ignition switch for the vehicle.

A reduction and non-essential accessory gear drive assembly 22 iscoupled to receive rotational energy output from automatic clutch 20 anddrive the service clutch 24. Gear drive assembly 22 also providesmechanical energy for driving non-essential vehicle accessories whichmay include an air conditioner compressor, an hydraulic pump, alubricant cooling fan, and an air compressor by way of example. Otheraccessories may of course be provided as required by the nature and useof the vehicle.

Service clutch 24 is a conventional, operator controllable clutch whichselectively couples the transmission to the engine. In some vehicleshaving an automatic transmission, the service clutch may not berequired. In other vehicles, such as a standard farm tractor, use of aservice clutch 24 may be desirable even in conjunction with an automatictransmission.

An infinitely variable transmission 26 is coupled to receive rotationalenergy from service clutch 24 when selectively engaged and outputrotational energy at a variable torque ratio to the primary vehicledrive system. For example, infinitely variable transmission 26 may becoupled to drive selected wheels of a vehicle through either a fixed orselectively variable discrete gear ratio. Infinitely variabletransmission 26 is shown as providing the vehicle control system anactual ground speed indication signal (G1). For a fixed gear ratiobetween the output of transmission 26 and the vehicle drive wheels,signal G1 is directly proportional to the rotational velocity at theoutput of transmission 26. In the event that transmission 26 is coupledto drive a variable gear ratio system, it would be necessary to eithergenerate signal G1 from some other location or modify signal G1 inaccordance with the selected gear ratio. Infinitely variabletransmission 26 provides a gear ratio which is variable over a finite,but continuous ratio spectrum in proportion to a ratio signal (R)generated by vehicle control system 12. Transmission 26, which may beconventional in nature, has a response time which is relatively fast incomparison with the response time of the vehicle control system 12. Theerror between the gear ratio commanded by signal R and the actual gearratio is therefore very small (less than 5%) and the vehicle controlsystem assumes that the actual gear ratio is identical to the commandedgear ratio. In the event that an infinitely variable transmission 26 isemployed in which substantial differences may develop between thecommanded and actual gear ratios, it may be desirable to provide thevehicle control system 12 with an additional input signal whichindicates the actual gear ratio of transmission 26.

The engine speed control 28 operates in a manner analogous to a throttleon a conventional manually controlled vehicle. Associated with theengine speed control is a manual-automatic switch which generates an M-Asignal which indicates an automatic mode of operation when the enginespeed control 28 is positioned in an inactive position and indicates amanual mode of operation when the engine speed control is actuated. Whenactuated, the engine speed control generates an engine speed commandsignal (N1 set) and the vehicle control system 12 operates to maintainengine speed at the commanded speed if possible. The ground speedcontrol 30 may be operated either independent of or in conjunction withthe engine speed control 28. Ground speed control provides a signal G1set in proportion to the positioning of the ground speed control 30. Inthe manual mode of operation, signal G1 set operates substantially as agear ratio command signal. Infinitely variable transmission 26 andvehicle control system 12 operate to provide a gear ratio and hence aground speed in proportion to signal G1 set. The transmission ratio andhence ground speed are automatically reduced if the engine cannot supplysufficient power to maintain the commanded ground speed at the enginespeed which is commanded by the engine speed control 28.

In an automatic mode of operation, the engine speed control 28 isdeactivated and vehicle operation is controlled solely by the groundspeed control 30. In this automatic mode of operation, the G1 set signaloperates solely as a ground speed command signal. If sufficient power isavailable, vehicle control system operates to accelerate the vehiclealong a predetermined acceleration curve to the commanded ground speed.In the automatic mode of operation, vehicle control systemsimultaneously and interactively controls the transmission ratio andengine velocity to permit the engine to operate near its most efficientoperating point for a given load condition. No attempt is made tomaintain any particular engine velocity. In the event that sufficientpower is not available the vehicle control system permits the actualvehicle speed to be decreased by as much as 50% of the commanded speed.If further speed reductions are required in view of available enginepower, an overload condition is allowed to occur. This speed reductionlimit is a safety feature which prevents inadvertent advancement of theground speed control 30 to a relatively high commanded speed while thevehicle is operating at a relatively low speed due to a heavy loadcondition. If such a great difference between commanded and actualground speed were allowed to occur, the vehicle might suddenly andrapidly accelerate toward the commanded speed in the event that the loadwere removed. If the vehicle were in a potentially dangerous situationwhere the operator anticipated and required continued low vehicle speed,this sudden acceleration might cause an accident. The speed reductionlimit thus operates to insure that sudden speed changes of more than 2:1cannot occur without manipulation of the ground speed control 30 by theoperator.

Other operator interface elements include the load meter 32 and thespeedometer 34. The speedometer 34 is a conventional ground speedindicator which operates in response to signal G1. Load meter 32 is arelatively sophisticated instrument which provides indications of enginespeed and engine load in combinations that depend upon particularvehicle operating conditions. This instrument is explained in greaterdetail below.

Referring now to FIG. 2, portions of the vehicular engine power system10 are shown in greater detail to permit a complete understanding of theinvention. For purposes of clarity and simplicity, some conventionalfeatures of a vehicular control system such as engine start-up and idlecontrols have been omitted or simplified. However, it should be assumedthat such conventional features are present in the control system eventhough they are not explicitly shown. Furthermore, gain modificationelements such as operational amplifiers have not been explicitly shown.However, it will be appreciated by one of ordinary skill in the art thatconventional gain control elements may be added to the signal paths asrequired to provide proper matching of signal amplitudes.

In a manual mode of operation, an engine speed lever within engine speedcontrol 28 is advanced and an engine speed set signal 35 is generatedwhich is proportional to the lever advance position. The engine speedset signal is summed with a temperature speed control signal 36 togenerate an uncompensated engine speed control signal 37. A lagcompensator 38 receives the uncompensated engine speed command signal 37and generates an engine speed command signal 39. The lag compensator 38closely matches the time rate of change of the engine speed commandsignal 39 with the acceleration capability of engine 14 for betterstability. In a manual mode of operation, a mode control switch 40,which may be a relay or electronic switch, is maintained in an opencondition by signal M-A and the engine speed command signal 39 issubstantially representative of the position of advancement of an enginespeed control lever within engine speed control 12.

A negative feedback loop is completed for conrol of engine speed bysubtracting the actual engine speed signal N1 from the engine speedcommand signal 39 to generate an uncompensated engine speed error signal42. A compensator element 44 modifies the uncompensated engine speederror signal 42 with either proportional gain or preferably proportionalgain plus the time integral of the uncompensated engine speed errorsignal 42 to generate an N1 ERROR signal which actually controls enginespeed by controlling the amount of fuel supplied to engine 14 undernormal operating conditions.

A lowest signal selector 50 receives a plurality of different controlsignals including the N1 ERROR signal and passes on the one of saidcontrol signals which is of smallest magnitude as a fuel control signal52 through a highest signal selector 53 to engine 14. The N1 ERRORsignal provides the normal engine control and the other signals are of asafety or precautionary nature. For example, an F1 functional element 54receives tachometer signal N1 and generates a maximum fuel signal 55 inaccordance with a predetermined maximum fuel schedule which limits therate at which fuel can be supplied to the engine 14 at any given enginespeed, N1. For example, since the maximum fuel schedule is a limit andnot a normal control it may be implemented with a circuit as shown inFIG. 3 with operational amplifiers 56a and 56b connected as invertingsumming and amplifier circuits, respectively. Since summing amplifier56a drives feedback resistor R1 with a voltage maintaining node 57 atground potential, the output voltage is V56aout=-(N1+VF1). The maximumfuel signal 55 is then V56bout=(RFb/R2) (N1+VF1). A more complex maximumfuel schedule may of course be developed if desired.

A temperature limited fuel signal 58 is generated by an F2 functionalelement 60 in response to engine temperature signals. A temperaturesensor 64 senses both exhaust gas temperature and air intake temperatureto generate the engine temperature signal T5 which generally representsexhaust gas temperature and signal T1 which represents intake airtemperature. The engine temperature signal T5 may be reduced somewhat bysignal T1 as the sensed air intake temperature increases to permit theengine 14 to run slightly hotter on a hot day. The F2 functional element60 prevents overheating of the engine 14 by subtracting a signal V1250,which indicates a temperature of 1250° F. from the adjusted exhaust gastemperature signal T5ADJ. The difference is then subtracted from a lowtemperature output voltage V1ow only when the difference is positive.For example, the temperature limited fuel signal 58 might remain at amaximum value, V1ow, until signal T5ADJ indicates an adjusted exhaustgas temperature of 1250° F. As the exhaust gas temperature signal T5ADJcontinues to increase the temperature limited fuel signal 58 might thenbe proportionately decreased to cut off all fuel supply to the engine 14as the engine temperature signal increases to indicate an adjustedexhaust gas temperature of 1300° F.

The circuit for F2 functional element 60 is shown in FIG. 4. Anoperational amplifier 58a is coupled to receive the inlet gastemperature signal T1 and generate an output signal having a voltageV58aout=-T1 (R4a/R4) which is used to adjust exhaust gas temperaturesignal T5. The adjustment is determined by the gain, R4a/R4. Forexample, if this gain is 1/2 as assumed for this example, exhaust gastemperature T5 would be permitted to increase 1/2 degree for each onedegree increase in inlet gas temperature T1.

An operational amplifier 58b is coupled as a summing amplifier togenerate an output voltage -T5ADJ=-(T5+V58aout)=-(T5-T1 (R4a/R4)).Operational amplifier 58c is also coupled as a summing amplifier togenerate an output voltage V58cout=-(T5ADJ+V1250) R4c/R4=R4c/R4(-T5ADJ-V1250) if diode D4 is temporarily ignored. Voltage V1250 isselected to equal signal T5ADJ at an adjusted exhaust gas temperature of1250° F. where fuel cutback is to begin. The gain R4c/R4 is selected tocause output voltage V58cout to change by a voltage equal to a voltageV1ow as signal T5ADJ changes in response to an exhaust gas temperaturechange of 50° F. Diode D5 substantially blocks the output V58cout unlesssignal T5ADJ indicates an exhaust gas temperature greater than 1250° F.

Another operational amplifier 58d is also coupled as a summing amplifierto generate temperature limited fuel signal58=-(V58cout-V1ow)=V1ow-V58cout. At low exhaust gas temperatures outputV58cout is approximately zero and temperature limited fuel signal58=V1ow to permit a substantial fuel flow. If the adjusted exhaust gastemperature increases above 1250° F., signal 58=V1ow-V58cout=V1ow-R4c/R4(-T5ADJ-V1250.

The highest signal selector 53 receives the fuel control signal 52,which is the normal fuel flow control signal, as well as a minimum flowschedule signal and passes on the highest of the two signals as amodified fuel control signal to control fuel flow. The minimum flowschedule signal from F1A functional element 65 enables adequate fuelflow for start-up and idle. Although the function may be morecomplicated if desired, the minimum flow schedule signal may increaselinearly with engine speed and be decreased somewhat as T1 increases(mass flow decreases). A typical functional relationship would be

    MFS=(M) (N1)+P (Q-T1)=(M) (N1)+(P) (Q)-(P) (T1)

where MFS is the minimum fuel schedule signal and M, P and Q areconstants which are selected for best performance of the particularcombustion and engine assembly. F1A functional element 65 may besuitably implemented with summing and amplifier circuits similar tothose for implementing the F1 functional element 54 which are shown inFIG. 3.

An emergency shutdown element 66 monitors conditions which mightpermanently damage the engine and generates an emergency fuel controlsignal 68 which is normally high but drops to zero to shut down theengine in the event that an emergency condition is detected. Forexample, the emergency shutdown element 66 might cause engine shutdownin the event that exhaust gas temperature gets too high, in the eventthat a proper starting sequence does not occur, in the event of a 110%engine overspeed condition, in the event of an oil pressure loss, or inthe event of some other emergency condition that is deemed desirable tomonitor. The emergency fuel control signal 68 is communicated to asolenoid valve 69 which is connected to interrupt the engine fuel supplywhen deactivated by the emergency fuel control signal 68 going low.

The engine speed control 28 thus operates in a manual mode to controlengine speed substantially independent of any other vehicle conditions.The engine speed control might be advantageously used to supply powerwhile a vehicle is at stand still for applications such as a power takeoff on a farm tractor or for a dump lift on a dump truck. However, withthe ground speed control 30 simultaneously activated, the engine speedcontrol 28 may also be utilized to modulate vehicle speed by controllingengine speed and hence engine power in a manner quite similar to thespeed control provided by the throttle of an ordinary automobile withautomatic transmission. For a single shaft gas turbine engine, theoutput power and torque of the engine reach a maximum at a rated 100%speed and decrease rapidly as the engine speed increases above ordecreases below the 100% rated speed. Good control over engine speed isthus very important for control of a vehicle having a single shaft gasturbine engine.

If the ground speed control 30 is actuated, for example by advancementof an operator controllable lever arm, while in a manual mode ofoperation, a ground speed set signal 76 is generated in proportion tothe lever position. This signal operates as a transmission ratioselector in the manual mode. The ground speed set signal 76 iscommunicated to an F3 functional element 78 which responds by generatinga ground speed command signal 80. F3 functional element 78 allows theground speed command signal 80 to approximately follow the ground speedset signal 76 except that the ground speed command signal 80 ispermitted to increase in magnitude only as a ramp function with apredetermined slope. The ramp slope is adjusted to be commensurate withthe ability of the engine 14 to accelerate the vehicle, and so long asthe power capability of engine 14 is not exceeded, determines the rateat which vehicle ground speed increases. FIG. 5 illustrates a circuitwhich will provide the suggested signal relationship. When the voltageof the set signal changes diode Z3A or Z3B causes the output ofamplifier A3A to act as a reference voltage across resistor RS3 or RS3+,depending on polarity. The output of amplifier A3B then changes with aslope of dV/dt=(ZV-0.6/RS3C). RS3+ determines the slope of a positivegoing ramp while RS3- determines the slope of a negative going ramp. Bychoosing (RS3-)(C) sufficiently small, the command signal 80 can followchanges in the set signal 76 substantially instantaneously. ZV is theZener breakdown voltage plus a forward diode voltage drop of diodes Z3Aand Z3B. Amplifier A3C merely provides negative feedback in an outerloop so that the command signal 80 can follow set signal 76 with a gainof RF3/R3 under steady state conditions.

The ground speed command signal 80 is communicated to a summing junction82 and a negative feedback loop for control of transmission ratio iscompleted by subtracting a signal R which is proportional to andindicates transmission ratio. In the absence of other factors the groundspeed command signal thus increases with a predetermined ramp whenground speed control 80 is actuated and the control loop causes thetransmission ratio to increase approximately along the ramp commanded bythe ground speed command signal 80. If the engine speed is permitted toremain constant during this ramping period the vehicle ground speed willalso accelerate in approximate conformity to the ramp. However, thevehicle may be pulling an unusually heavy load or the engine speedcontrol may be set at a position which does not permit full engine powerto be developed. Under such circumstances there may not be sufficientengine power available to permit the vehicle to accelerate along theramp commanded by the ground speed command signal 80. In the absence ofother control signals the engine 14 would be unable to meet the powerdemand and would stall.

However, additional negative feed back is provided to summing junction82 to reduce the transmission ratio and thus the power demand on theengine 14 when the demanded power exceeds that which the engine 14 cansupply. An F4 functional element 84 responds to the engine speed commandsignal 38 by generating a modified engine speed command signal 86.Sudden reductions in the transmission ratio are avoided for purposes ofstability by permitting the modified engine speed command signal 86 toincrease only along a ramp function. The modified engine speed commandsignal 86 can rapidly follow a step function decrease in the enginespeed command signal 39. Construction of the F4 functional element 84may be substantially the same as the F3 functional element 78 which isshown in FIG. 5. A summing junction 90 receives the modified enginespeed command signal 86 as a positive input and the actual engine speedsignal N1 as a negative feedback input and generates a transmissioncontrol engine speed error signal 92 as an output.

An F5 functional element 94 receives the transmission control enginespeed error signal 92 and generates a ratio reduction signal 96 inresponse thereto. The ratio reduction signal is never permitted to gonegative so that it cannot tend to increase the transmission ratio andfor reasons of stability, the ratio reduction signal 96 follows thetransmission control engine speed error signal 92 only when thetransmission control engine speed error signal 92 is positive andexceeds a predetermined threshold magnitude. The ratio reduction signal96 otherwise has a zero magnitude and does not affect the transmissionratio. A possible implementation of F5 functional element 94 is shown inFIG. 6. When signal 92 exceeds the threshold the Schmidt trigger closesswitch SW5. It has been found that adequate stability and good responsecharacteristics are attained when the threshold magnitude is set atapproximately 2%. That is, the ratio reduction signal 96 becomes activewhen the actual engine speed N1 becomes less than 98% of the commandedengine speed as indicated by the modified engine speed command signal86. Thus, when the load demanded of engine 14 exceeds its powercapabilities, engine speed is reduced below that which is commanded andthe ratio reduction signal 96 is generated to reduce the transmissionratio and thereby decrease the load demand.

Actual transmission ratio is commanded by a transmission ratio signal Rwhich is generated as the output of a maximum signal selector 100. Undernormal circumstances signal R is generated by linear amplification of aratio error signal 102 which is generated as the sum of the inputs tosumming junction 82. Maximum signal selector 100 is a circuit whichreceives a plurality of inputs and generates the one input with thelargest magnitude as the output. An actuator within transmission 20 isable to follow a commanded transmission ratio R with a speed which israpid compared to the acceleration rate for engine 14 and the signal Ris taken as an accurate representation of both commanded and actualtransmission ratio.

In an automatic mode of operation the engine speed set signal 30 remainsat zero and switch 40 remains continuously closed to permit thetemperature speed control signal 36 to command engine speed. Thetemperature speed control signal 36 is generated by an F6 functionalelement 120 in response to adjusted exhaust gas temperature signal-T5ADJ which may be derived as in F2 functional element 60.

The F6 functional element 120 may take any one of several possiblearrangements which automatically control engine operation to attain goodfuel economy by keeping the engine 14 operating with an exhaust gastemperature at or near the maximum temperature of approximately 1250° F.In a first functional relationship which is illustrated as curve F6 inFIG. 7, the temperature speed control signal 32 is clamped at 60% whenthe adjusted exhaust gas temperature signal indicates an exhaust gastemperature at or below 600° F. Under this circumstance the engine 14 iscaused to idle at 60% of its rated speed. As the adjusted exhaust gastemperature signal increases above an indication of 600° F. thetemperature speed control signal 36 is increased proportionately up to amagnitude sufficient to command a maximum speed of 100% rated speed asthe adjusted exhaust gas temperature signal reaches a maximumpermissible temperature of approximately 1250° F. Minimum basic specificfuel consumption (optimum fuel efficiency) occurs when exhaust gastemperature is at a maximum. This first arrangement for the F6functional element permits the engine to be automatically operated nearthe high exhaust gas temperature for good fuel consumption when undersubstantial load.

If an increased load is applied to the engine 14 while at idle or underpartial load, the exhaust gas temperature increases, and an increasedoperating speed is commanded. As the commanded operating speed exceedsthe actual operating speed an N1 error signal is developed which causesmore fuel to be supplied to the engine. The engine responds byaccelerating until the exhaust gas temperature decreases to cause adecrease in the commanded operating speed. The engine thus seeks anincreased operating speed where the increased power output can match theincreased power demand.

A circuit for generating the functional relationship of F6 functionalelement 120 is shown in FIG. 8. Operational amplifier 121 is coupled ina summing amplifier configuration to generate an output voltage

F6out=-RF6/R6(-V60%+(-T5ADJ+V600))=RF6/R6(V60%+T5ADJ-V600)

when adjusted exhaust gas temperature is between 600° F. and 1250° F.Below 600° F. -T5ADJ has no effect on circuit operation because diode D6is reverse biased and the output voltage F6out is clamped atF6out=(RF6/R6)V60%. Above 1250° F. resistor RL6 and Zener diode ZD7operate to clamp the output voltage at the Zener breakdown voltage whichshould be selected to indicate 100% engine speed. The circuit gainRF6/R6 should be selected to cause the output to change from 60% to 100%engine speed command as -T5ADJ changes from an indication of 600° F. toan indication of 1250° F. Voltage -V60% should be selected in view ofthe circuit gain to cause a minimum 60% output signal and the voltage ofbattery B6 plus the forward conduction voltage drop of diode D6 equalV600 should equal the voltage of signal -T5ADJ at an adjusted exhaustgas temperature of 600° F.

By modifying the proportional curve F6 to make it somewhat steeper asrepresented by the curve F6' shown in FIG. 7, the engine operating speedcan be biased toward an even higher temperature (lower speed) operatingpoint with slightly better fuel efficiency. In this second configurationof F6 functional element 120, the gain RF6/R6 is increased and voltageV600 is increased to equal signal -T5ADJ at a temperature of about 1200°F. This second arrangement tends to improve steady state part load fuelconsumption efficiency by operating the single shaft gas turbine engine12 even closer to the exhaust gas temperature limit under part loadconditions. However, the higher gain decreases operating point stabilityand the engine 14 becomes more subject to acceleration and deceleration.

Still a third arrangement which offers a compromise between the higherefficiency under moderate load provided by curve F6' and the betterstability of curve F6 is illustrated by curve F6" in FIG. 7. Thisfunctional relationship can be approximated by substituting the circuitshown in FIG. 9 for battery B6 and diode D6 in FIG. 8. An operationalamplifier 121a is coupled as an inverting amplifier to change the gainof signal -T5ADJ, which is normalized by battery B6 and diode D7 topresent a negative voltage at the input to amplifier circuit 121abeginning at zero volts as adjusted exhaust gas temperature increasesbeyond 600° F. The gain, RF6a/R6, of amplifier 121a is selected togenerate an output voltage less than or equal to about 1.2 volt at atemperature of 1250° F. The output of amplifier 121a is coupled throughtwo diodes D6a and D6b to the input of another inverting amplifiercircuit 121b. Diodes D6a and D6b pass a circuit therethrough which hasan expotential relationship to the output voltage from amplifier 121a.Feedback resistor RF6b controls the gain of amplifier 121b, whichgenerates an output voltage that is proportional to the current throughdiodes D6a and D6b. Resistor RF6b should be selected to produce anoutput voltage that is coincident with the voltage at the anode of diodeD6 in FIG. 8 when signal -T5ADJ indicates 1250° F. Regardless of whicharrangement is selected for F6 functional element 120, the automaticcontrol operates the engine 14 at a speed which will maintain arelatively high exhaust gas temperature and provide good fuel efficiencyfor a given load condition. This is in contrast to manual constant speedoperation in response to the engine speed control 28 wherein a commandedengine speed is followed without regard to fuel efficiency.

Control over the transmission is substantially the same in the automaticmode of operation as in the manual mode of operation except that aswitch 130 is closed in response to the automatic indication of the M-Asignal to add an outer ground speed error loop into the transmissionratio control system. This is accomplished by subtracting the G1 actualground speed signal from the ground speed set signal 76 and adding thedifference as a positive input 132 to summing junction 82. The magnitudeof the resulting G1 ERROR signal is maintained relatively low incomparison to the magnitude of the ground speed command signal 80 andthe ratio reduction signal 96 so that the G1 ERROR signal has minimaleffect upon the transmission ratio R during transient operatingconditions. Once these other signals become approximately balanced withthe transmission ratio signal R, the G1 ERROR signal operates as a finetuning signal to provide somewhat more precise control over actualground speed.

An F7 functional element 134 cooperates with the maximum signal selector100 to implement a special safety feature associated with the vehiclepower system 10. Under a full load condition the vehicle ground speed iscontrolled by adjusting the transmission ratio to match the vehicle loadwith available power. For example, a tractor plowing hard ground mighthave the ground speed control 30 set for a ground speed of 18 miles perhour while the available power would permit a ground speed of only onemile per hour. Under such circumstances it might be possible for anoperator to think the ground speed control 30 has been set for 3 milesper hour when it has actually been unknowingly bumped by the operatorand set to 18 miles per hour. Under such circumstances if the load isremoved from the tractor, for example, by lifting the plow out of theground at the end of a furrow, the tractor would accelerate at maximumrate towards 18 miles per hour. This sudden and unexpected accelerationmight cause the tractor operator to lose control and if the tractor isnear a ditch or a building serious damage might result before thetractor is brought back under control.

An under speed limit circuit avoids this possibility by providing atransmission ratio override signal 136 in response to the ground speedcommand signal 80. Ratio override signal 136 commands a transmissionratio R which will result in an actual ground speed of approximately 50%of the ground speed indicated by the ground speed command signal 80 at100% engine speed. The maximum signal detector 100 operates to permitthe override signal 136 to become operative only when its magnitudeexceeds the signal R which is generated in response to ratio errorsignal 102. A circuit for implementing F7 functional element 134 may beimplemented with two equal resistors and a diode coupled successivelybetween signal 80 and ground with signal 136 taken from the commoncoupling of the resistors. The diode provides a small offset to improveoperating characteristics near zero vehicle velocity. Under normalcircumstances the engine 14 has sufficient power to maintain the vehiclespeed at more than half the commanded speed and the under speed limitcircuit has no effect on vehicle operation. However, if the vehicleencounters a large load the ground speed cutback which results fromreduction of the transmission ratio R will be limited by the under speedlimit circuit to 50% of the command ground speed. Under suchcircumstances the engine 14 will be unable to develop sufficient torqueto overcome the load and will decelerate toward a stall condition unlessthe commanded vehicle speed is reduced.

If infinitely variable transmission 26 is of the non-slip type having aratio which is infinitely variable down to R=0, use of the serviceclutch 24 is required for stopping. Furthermore, the possibility of aninfinite torque ratio between the engine and drive wheels must beprevented. For such a transmission the input of a fixed voltage signal RMIN to maximum signal selector 100 places a lower limit on theattainable transmission ratio. This ratio should be selected such thatapplication of vehicle service brakes to halt a vehicle withoutdisengagement of the service clutch would cause engine 14 to overloadand stall before mechanical damage is sustained by the power train.

An additional feature of the vehicle power system 10 is the automaticclutch 20 that is controlled in response to a clutch control signal 152which is generated by an automatic clutch control circuit in response tothe actual engine speed signal N1. Any time the signal N1 indicates anactual engine speed below 55% of rated speed, which is very near the noload engine stall speed, the automatic clutch control circuit 154 sensesthis low signal condition and generates a clutch control signal 152which commands disengagement of the automatic clutch 150. Disengagementof automatic clutch 20 removes most of the load from the engine 14 andpermits the engine to accelerate to the 60% normal idle speed. A memorycircuit, such as a flip-flop, within the automatic clutch controlcircuit 154 becomes set any time the clutch control signal 152disengages the automatic clutch 20 and remains set to inhibit the clutchcontrol signal 152 from commanding reengagement of the automatic clutchuntil the memory circuit is reset by the clutch reset signal. This resetrequirement prevents a limit cycle condition wherein the automaticclutch is repeatedly disengaged as the engine 14 accelerates above ordecelerates below the 55% cutoff point. The clutch reset signal may beadvantageously generated by an ignition-starter switch which permits theautomatic clutch control to be automatically reset each time the engine14 is started. The starter system may be disabled in response to signalN1 to prevent a starter response to actuation of the ignition switch togenerate the reset signal when the engine is idling followingdisengagement of automatic clutch 20. The automatic clutch prevents atotal loss of power due to engine stall, eliminates a loss of timerequired for restarting the engine, and reduces the wear and tear on thevehicle battery and starter system which may result from repeatedstart-ups.

As shown in FIG. 10, the load meter 32 includes a conventionalD'Arsonval movement panel meter 180 having a pointer 182 with arotational position controlled by a rotating mechanism 184 which causesthe rotational position to be proportional to current supplied by acurrent driver 186. Current driver 186 generates a current which isproportional to a load signal 188 from a switch 190. Switch 190 respondsto the M-A (Manual Automatic) mode signal to connect the load signal 188to the N1 engine speed during a manual mode of operation. Load meter 32thus serves as a standard tachometer to indicate engine speed over arange of 0 to 100% of rated speed in the manual mode of operation.

In an automatic mode of operation, switch 190 is repositioned to providethe load signal 188 from the output of a summing junction 192. Summingjunction 192 receives as inputs the ground speed error signal, G1 ERROR;the engine speed error signal, N1 ERROR; and a clamped engine speedsignal, N1C. During part load and full load engine operating conditions,a switch 194 is positioned by an actuator 196 to provide the summingjunction 192 with a clamped engine speed signal equal to the actualengine speed signal, N1. Thus, under part load conditions, the loadmeter 32 operates as a tachometer to indicate engine speed in a mannersubstantially identical to the manual mode of operation.

As load is increased, the automatic control system causes the enginespeed to increase to 100% rated speed and the indicator 182 is rotatedto a marker angle 198 to indicate this engine speed. As a further loadis placed on the vehicle, the power output of engine 14 cannot beincreased and the additional load is compensated by reducing thetransmission ratio to provide a ground speed less than the commandedground speed. This ground speed cutback causes generation of a voltageon the G1 ERROR signal which is added by summing junction 192 to the100% N1 engine speed signal to cause the indicator 182 to rotate furtherin a clockwise direction beyond the 100% engine angle 198 to an overloadindication angle 200. The gain of the G1 ERROR signal is selected incooperation with the positioning of overload angle indication 200 insuch a manner that the indicator 182 rotates to the overload position atangle 200 as the ground speed cutback reaches 50% of the command groundspeed.

At this point, the F7 functional element 134 and the maximum signalselector 100 operate as shown in FIG. 2 to limit any further groundspeed cutback. Further increases in the vehicle load must then cause theengine 14 to overload and begin slowing down. As the engine 14 slowsdown, the engine speed becomes less than the 100% speed commanded by theautomatic control system 12 and the engine speed error signal, N1 ERROR,begins to increase in magnitude. As signal N1 ERROR exceeds a relativelysmall threshold which is normally required for activation of an enginefuel valve at 100% engine speed, actuator 196 senses this increase andrepositions switch 194 to clamp signal N1C at a voltage equal to thevoltage of signal N1 at 100% engine speed. Summing junction 192 thusreceives a clamped engine speed signal N1C, which has a magnitudesufficient to drive indicator 182 to the 100% indicator position 198;plus a G1 ERROR signal which has a magnitude sufficient to drive theindicator 182 beyond the 100% engine speed indication 198 to an overloadindication position 200; plus an N1 ERROR signal which is added to thesignal N1C and G1 ERROR to drive the indicator 182 clockwise beyond theoverload indication 200 toward a stall and automatic clutch indicationposition 202. The gain of the N1 ERROR signal and the positioning of thestall and automatic clutch position 202 are selected such that indicator182 rotates to the stall and automatic clutch indication position 202 asthe engine decelerates toward the stall velocity under an overloadcondition. This would typically occur at an engine velocity atapproximately 55% which would be equal to an engine speed error of 45%.Indication angle 202 is the point at which the automatic clutch 20 wouldnormally be disengaged to prevent a complete engine stall. A redindication area 204 is provided counterclockwise of the stall andautomatic clutch indication position 202 to indicate further overloadmagnitudes in the event that automatic clutch 20 is not disengaged.Movement of indicator 182 into the red zone 204 indicates that theengine velocity has decreased below a self-sustaining velocity and thatengine stall is inevitable.

For convenience of the operator, additional color coded zones may beprovided on the panel meter 180 and engine speed markings may beprovided at shorter intervals than are shown in the 0 to 100% speedrange. For example, a start zone 206 may be colored white between 0 and40% speed indication angles. At 15% of rated speed, fuel is typicallyturned on during start-up or turned off during shut down. A fuel zone208 may thus be established between the 15% fuel off speed and the 55%stall and automatic clutch speed and colored red to indicate a zonewithin which the fuel is on but engine velocity is not self-sustaining.This red zone complements the overload red zone 204 which is indicatedwhen engine speed drops below a self-sustaining speed while engine 14 isunder load. A yellow zone 210 may be conveniently provided betweenindications of the 55% stall and automatic clutch speed and the 60%normal idle speed. A speed range 212 between the 60% idle speed and 100%speed is the normal operating speed range for engine 14 and mighttypically be colored green.

While there has been shown and described a preferred arrangement of avehicular single shaft gas turbine engine power system in accordancewith the invention for the purpose of enabling a person of ordinaryskill in the art to make and use the invention, it will be appreciatedthat the invention is not limited thereto. Accordingly, anymodifications, variations or equivalent arrangements within the scope ofthe attached claims should be considered to be within the scope of theinvention.

What is claimed is:
 1. A load meter for a vehicular gas turbine enginecomprising a magnitude indicator connected to indicate (1) a magnitudewhich is directly proportional to engine operating speed within a firstmagnitude range which extends between a first low indicated magnitudeand a second indicated magnitude greater than the first indicatedmagnitude, (2) a magnitude which is proportional to the sum of (a)engine operating speed and (b) a ground speed error which represents thedifference between commanded and actual vehicular ground speed within asecond magnitude range which extends between a third indicated magnitudewhich is greater than or equal to the second indicated magnitude and afourth indicated magnitude which is greater than the third indicatedmagnitude, (3) a magnitude which is proportional the sum of (a) engineoperating speed, (b) ground speed error and (c) an engine speed errorwhich represents a difference between actual engine speed and enginespeed at a maximum torque operating condition within a third magnituderange which extends between a fifth indicated magnitude which is greaterthan or equal to the fourth indicated magnitude and a sixth indicatedmagnitude greater than the fifth indicated magnitude.
 2. The load meteras set forth in claim 1 above, wherein the second and third indicatedmagnitudes are substantially the same and indicate an engine operatingspeed at which maximum torque is developed.
 3. The load meter as setforth in claim 1 above, wherein the second magnitude range indicates arange of operation wherein engine speed is maintained at approximately amaximum torque operating speed as ground speed error is increased inresponse to a decrease in a ground speed to engine speed transmissionratio which is implemented to maintain the engine speed at a maximumtorque operating speed as opposition to vehicular motion increases. 4.The load meter as set forth in claim 3 above, wherein the fourthindicated magnitude represents a vehicle operating condition wherein thetransmission ratio has been reduced to a point where ground speed errorhas reached a predetermined maximum beyond which further reductions inthe transmission ratio are not permitted.
 5. The load meter as set forthin claim 4 above, wherein the fifth indicated magnitude is substantiallyequal to the fourth indicated magnitude and wherein the third magnituderange represents a range of operation wherein the engine is unable todevelop sufficient torque to drive the load thereon and continuedmaintenance of the load on the engine necessarily results in an increaseof engine speed error and an increase in magnitude indicated by the loadmeter.
 6. The load meter as set forth in claim 5 above, wherein the loadmeter indicates a seventh magnitude within the third magnitude rangebeyond which engine operating speed is too slow to prevent engine stalleven if the load is removed from the engine.
 7. The load meter as setforth in claim 1 above, wherein the magnitude indicator comprises apointer adapted to rotate about an axis with the magnitude of angulardisplacement from a predetermined zero position being proportional to anapplied signal.
 8. A load meter for a vehicle powered by a gas turbineengine comprising an indicator connected to indicate engine operatingspeed when the engine is operating at less than full engine power andindicating means connected to indicate a difference between commandedvehicle performance and actual vehicle performance when the engine isoperating at full engine power.
 9. The load meter as set forth in claim8 above, wherein the vehicle has a commanded speed selector, wherein thevehicle has an infinitely variable transmission which may be controlledto continuously vary a transmission ratio of ground speed to enginespeed, wherein the transmission ratio is reduced when engine powerrequired to maintain a commanded vehicle speed is greater than maximumavailable engine power until a difference between actual and commandedvehicle speed reaches a predetermined magnitude, and wherein the loadmeter is arranged to indicate two separate operating ranges when theengine is operating at maximum power, one maximum power range beingindicated when the difference between commanded and actual vehicle speedis less than the predetermined magnitude and a second maximum powerrange being indicated when the difference between commanded and actualvehicle speed is greater than the predetermined magnitude.
 10. The loadmeter as set forth in claim 9 above, wherein the load meter indicatesthe difference between actual engine speed and engine speed when maximumengine power is being developed in the second maximum power range.